Updates to the btrfs file system utilities that include new
features, such as the skinny-metadata feature, are
incompatible with UEK R3. If the /root
partition is formatted using btrfs, a system running UEK R3 is
unable to boot as it cannot mount the /root
file system. The system is able to boot using UEK R4 or RHCK
kernel types.

This problem is particularly important during installation, as
it is possible to manually configure partitioning and the file
systems from within the installer. When the file system is
formatted within the installer, using btrfs, all of the newer
features are enabled. If you select to downgrade the kernel to
UEK R3 after installation, the system is unable to boot.

Systems that are already using btrfs with UEK R3 are
unaffected when they are upgraded, as the file system would
have been formatted with the new features disabled. (Bug ID
24840489)